In certain sensory systems, natural sensory input has been to shown to play a crucial role in the formation of connections and functional activity of CNS neurons. However, this has not been demonstrated for the otolithic pathways since there is no simple way to deprive the system of gravitational stimulation on Earth. Thus, the effects of microgravity on central vestibular development can be studied uniquely in space flight. Here we will perform ground-based studies which will be the controls for our future space flight experiments. For this work, a simple and established avian model will be studied, the chick tangential vestibular nucleus. The principal cells of this nucleus are second-order vestibular neurons participating in the three-neuron vestibule-ocular and vestibulocollic reflexes, which are involved in coordinating head and eye movements. This laboratory has a long history performing combined structure/function studies to investigate neuronal excitability in identified vestibular nucleus neurons of older chick embryos and hatchlings. More is known about principal cell development than for any other class of vestibular nucleus neuron. However, little is established on their earliest functional development, and how it may be influenced by microgravity. The techniques include whole-cell patch clamp recording, pharmacological testing, immunocytochemistry combined with quantitative analysis of fluorescent confocal images, and low molecular weight tracer injections in brain slices of chick embryos (E5-E12) and intact hatching chicks. The specific aims include: (1) Describe the pathways, distribution, and terminals formed by otolithic afferents in the chick vestibular nuclei and their relationship to ampullary nerve inputs in hatchlings; (2) Study the onset and emergence of action potential generation in response to depolarizing current pulses and identify the underlying membrane conductance's, focusing on potassium channels; (3) Investigate gap-junction mediated intercellular communication among vestibular neurons and among glial cells; (4) Identify the source of axons forming the first non-vestibular inputs in the tangential nucleus (lateral vestibular nucleus).